1. Field of the Invention
The invention relates to a welding method and a welding structure, and more particularly, relates to a welding method and a welding structure of laser welding.
2. Description of Related Art
One widely used welding method for joining together a plurality of overlapped welding objects (such as metal plates, for example), is a welding method by laser welding, because it offers advantages such as that there is little processing distortion, high speed welding is possible, and few portions are affected by residual heat. With a laser welding method, a weld is typically formed in a straight line by scanning laser light in a straight line.
Here, stress concentrates at both end portions of the weld formed in a straight line. As a result, various problems, such as that this kind of weld tends to be defective which makes it difficult to achieve consistent weld quality, have been pointed out. Therefore, in order to solve these kinds of problems, a laser welding method that forms welds in a variety of shapes has been proposed.
For example, Japanese Patent Application Publication No. 2009-233712 (JP 2009-233712 A) describes a laser welding method that forms a weld in two “C” shapes arranged facing one another. With this kind of laser welding method, a curved portion of the weld that is a C-shaped arc is arranged on the end portions on both sides of a weld zone where stress tends to concentrate. On the other hand, a start point (i.e., a starting point where the laser starts to be emitted) and an end point (i.e., an end point where the laser stops being emitted) of the weld are arranged in an intermediate portion that is between the end portions on both sides and tends not to be affected by external force (does not contribute much to improving weld strength). As a result, the start point and the end point of the weld are not arranged at the end portions on both sides where stress tends to concentrate, so the concentration of stress at the weld is mitigated. Also, with the laser welding method in JP 2009-233712 A, the weld is formed in two “C” shapes that face each other, so welding of the intermediate portion that tends not to be affected by external force is omitted. That is, production efficiency is improved because a position that does not contribute much to an improvement of weld strength is not welded.
Here, with a welding structure formed by laser welding, there is tendency for stress to concentrate at a boundary between the weld and base material. Therefore, cracking may occur at the boundary of the weld and the base material. Moreover, if the boundary lies in the same direction as the direction in which the crack progresses, the crack will tend to progress further. If this happens, the welded objects that are joined together may fracture (i.e., break apart).
However, a weld formed by the laser welding method according to JP 2009-233712 A described above is no stronger than a weld formed in a circular shape or circumferential shape of the same size as the weld formed in two C shapes. Therefore, a weld formed by the laser welding method according to JP 2009-233712 A is no more effective against the problem described above than a weld formed in a circular shape or circumferential shape of the same size.